Inkjet Printing
Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines, which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Generally, there is a mechanism for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
Most number of such prints are produced in the home and office environment using small apparatus capable of printing on relative small areas only. Most popular paper formats are standard office formats such as the ISO 216 A4 paper size and the ANSI/ASME Y14.1 Letter format. Larger size printers usually can print on ISO 216 A3 or ANSI/ASME Y14.1 Tabloid format.
In all, these printers are limited in size and throughput.
In recent times e.g. inkjet printers have evolved to more industrial applications. A lot of these printers can handle larger paper formats or use special types of ink.
Preferably these industrial printers are capable of printing on large paper sized and obtain a high throughput. Sizes up to 200×280 cm are desirable as output format. Special applications are e.g. poster printing, advertising . . . To obtain a higher throughput usually several printhead are used at the same time.
To improve the clarity and contrast of the printed image, recent research has been focused to improvement of the used inks. To provide quicker, more waterfast printing with darker blacks and more vivid colours, pigment based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to forms high quality images. In some industrial applications, such as making of printing plates using ink-jet processes, inks having special characteristics causing specific problems.
E.g. UV curable inks exist to allow rapid hardening of inks after printing. An example can be found in WO 02/53383. A special UV source has then to be provided for curing the inks after printing. After the ink of a printed band has been partially cured by the UV source, the band can be immediately be overprinted without the problem that the ink drops will mix causing artefacts.
Using this ink allows for the use of high quality printing methods at a high speed avoiding several other problems inherent to the nature of the recording method.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots.
Certain artefacts such as Moiré patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots.
Another source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as the location of the marker elements in the head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the length of the print. A variation in drop velocity will also cause artefacts when the printing head is moving as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the way the printing medium is moved relative to the printing medium may result in defects which may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors. In fact, any geometrical limitation of the printing system can be a source of errors, e.g. the length of the printing head, the spacing between marking elements, the indexing distance of the printing medium relative to the head in the slow scan direction. Such errors may result in “banding” that is the distinct impression that the printing has been applied in a series of bands. The errors involved can be very small—the colour discrimination, resolution and pattern recognition of the human eye are so well developed that it takes remarkably little for errors to become visible.
To alleviate some of these errors it is known to alternate or vary the use of marker elements so as to spread errors throughout the printing so that at least some systematic errors will then be disguised. For example, one method often called “shingling” is known from U.S. Pat. No. 4,967,203 which describes an ink jet printer and method. Each printing location or “pixel” can be printed by four dots, one each for cyan, magenta, yellow and black. Adjacent pixels on a raster line are not printed by the same nozzle in the printing head. Instead, every other pixel is printed using the same nozzle. In the known system the pixels are printed in a checkerboard pattern, that is, as the head traverses in the fast scan direction a nozzle is able to print at only every other pixel location. Thus, any nozzle which prints consistently in error does not result in a line of pixels in the slow scan direction each of which has the same error. However the result is that only 50% of the nozzles in the head can print at any one time. In fact, in practice, each nozzle prints at a location which deviates a certain amount from the correct position for this nozzle. The use of shingling can distribute these errors through the printing. It is generally accepted that shingling is an inefficient method of printing as not all the nozzles are used continuously and several passes are necessary.
Another method of printing is known as “interlacing”, e.g. as described in U.S. Pat. No. 4,198,642. The purpose of this type of printing is to increase the resolution of the printing device. That is, although the spacing between nozzles on the printing head along the slow scan direction is a certain distance X, the distance between printed dots in the slow scan direction is less than this distance. The relative movement between the printing medium and the printing head is indexed by a distance given by the distance X divided by an integer. More sophisticated printing schemes can be found in e.g. European application EP 01000586 and U.S. Pat. No. 6,679,583.
Another problem is that high acceleration values are needed when the shuttle starts printing. Acceleration can be up to 10 m/s2. Lower acceleration values to reach high printing speeds would give less problems regarding vibrations but would lead to loss of time due to longer run-up time and inevitably longer run-up distance leading to even larger dimensions of the overall apparatus giving rise to more problems of stability.
Thus these industrial printers usually comprise:                large size recording units        use of multiple heads        heavier weight        high speed movements over long distances        higher accelerations        complicate recording schemes (shingling, interlacing, . . . )        large ink reservoirs with online replenishment of the ink tanks on the printhead shuttle.        
and can further also comprise                UV pre-curing installation        cooling means        cabling and ink transport tubes.        
To enable high quality recording a precise and reproducible positioning and control of the printing unit is needed in these industrial machines. For high quality printing the dot placement accuracy is set to about 5μ, while dots printed have a size in of about 30μ. However depending upon the application of the printer accuracy and dot size may vary.
The positioning systems used in the state of the art home and office printers can not be simply enlarged to be used in the industrial printing apparatus.
In JP20012701870 a method is provided for driving a carriage of an inkjet printer wherein the belt drive system has two motors, one stepping motor an done DC motor which is used during acceleration of the carriage.
In U.S. Pat. No. 5,365,839 use is made of a shuttle and a balance shuttle driven by linear motors.
Several Problems Arise:                inertia problems due to higher weight of printhead and utility components (UV source, . . . ).        bending of the frame due to gravitation or drive forces of the motor system.        torsion of large size spindles.        strain due to tension on the components of the shuttle drive system.        insufficient rigidity of the apparatus frame leading deformation due to stress forces and incorrect resulting in incorrect placement of dots and incorrect recording distance of the printhead over the receiver.        cost of a large high accuracy shuttle drive system, e.g. long stroke linear motors are very expensive.        
The large forces needed to drive the printing shuttle lead to vibrations giving printing defects as the reference points of the print head positioning system and the receiver positioning are not rigidly fixed to each other. It can be considered that the axis x of the co-ordinate system of the printhead drive and receiver are not locked to each other.
Certain industrial printers use a low number of printheads, keeping weight of the printing shuttle down, thus having the negative effect that throughput is very low.
Other types use more printheads but need a very expensive paper drive system to ensure accuracy.
Some industrial printers are only capable of low quality end products such as those used in large-size advertising boards.
It is clear that the state of the art driving mechanism of office printers are not capable of driving the large printing shuttles of industrial printers at the needed speed and accuracy.
It is clear that to obtain a high throughput, high quality industrial inkjet printing apparatus am improved printing shuttle has to be developed having high accuracy over a large area and capable to perform a high speeds and acceleration values.